A) Field of the Invention
The present invention relates to a method of forming a thin film pattern of noble metal such as platinum.
B) Description of the Related Art
A temperature sensor using a platinum thin film resistor is formed by depositing a platinum thin film on a substrate and pattering it in a fine wire shape to form a resistor. A change in the electrical resistance value of the platinum resistor is detected to measure the temperature.
The temperature characteristics of a platinum thin film are important characteristics. The temperature characteristics of a platinum thin film depend on the film qualities such as the diameter of crystal grains of the platinum thin film. The diameter of crystal grains is influenced by the thin film forming conditions.
A sensor of a thin membrane structure having a platinum resistor covered with an oxide film or the like has drawn attention because it has a small thermal capacity and a good response. An insulating film is formed on a silicon substrate, and a platinum resistor is formed on the insulating film. After the platinum resistor is covered with an insulating film, the silicon substrate under the platinum resistor is etched and removed. By removing the thermal conduction and thermal capacity of the silicon substrate, a temperature sensor of a high sensitivity and a high response can be obtained.
A temperature sensor of such a membrane structure is required to have a high mechanical strength of the membrane structure itself. Levels satisfying this requirements are necessary for working and finishing a platinum thin film pattern.
For example, it is necessary to prevent dusts and foreign matters from being attached to a platinum thin film pattern and prevent the underlying oxide film from being made too thin or damaged during patterning the platinum thin film. If the side wall of the platinum thin film pattern is vertical to the surface layer of the substrate, the quality of an insulating film such as an oxide film to be deposited on the platinum thin film pattern is likely to be degraded. The mechanical strength of the membrane structure is therefore lowered. It is desired to make the platinum thin film pattern have a trapezoidal cross section with tilted or sloped side walls.
The following two methods have been adopted to form a platinum thin film pattern.
With the first method, as shown in FIG. 3A, an insulating film 7 is formed on the whole upper surface of a substrate 1, and a platinum thin film 2 is deposited on the insulating film 7 by sputtering or the like. Resist is coated on the platinum thin film 2, exposed and developed to leave a resist pattern 3 in a necessary area. The resist pattern 3 is heated to 150 to 200° C. to fluidize it and form tapered side walls.
As shown in FIG. 3B, the platinum thin film 2 is milled in a milling system. A tapered side wall of the resist pattern 3 is exaggeratedly transferred to the platinum thin film. After the platinum thin film 2 is patterned, the resist pattern 3 is removed by ashing or the like. As shown in FIG. 3C, an insulating film 8 is formed on the substrate, covering the patterned platinum thin film 2.
As shown in FIG. 3D, the substrate 1 under the platinum thin film 2 is etched and removed. With this first method, since milling is a physical work utilizing impacts of ions having a high energy, hardened resist after milling and sputtered side wall residues 4 peculiar to milling (so-called rabbit ear) are left and are hard to be removed.
It is necessary to fix the substrate 1 with a clamp in order to perform milling. Clamping may damage the product and the manufacture yield may be lowered. The surface layer of the substrate is also milled. Depending upon the milling conditions, the insulating film 7 is excessively milled to form a groove-like recess 5 as shown in FIG. 3C. The product quality may be lowered, e.g., a lowered mechanical strength.
The degree of the taper or slope of a side wall of the platinum thin film pattern depends upon the taper or slope degree of the resist pattern side walls. As the width of the resist pattern becomes narrow, the slope degree of the resist pattern cannot be made large. If the temperature of post-baking the resist pattern is raised to increase the slope degree of the resist pattern side walls, it becomes difficult to remove the resist pattern.
With the second method, as shown in FIG. 4A, on an insulating film 7 formed on a substrate 1, a resist film 3 is coated. The resist film 3 in an area where a platinum thin film pattern is to be formed is removed. In this case, the resist film 3 is made sufficiently thick so that a platinum thin film to be deposited at the next process has stepwise cuts. In addition, a focus of resist exposure is shifted intentionally to broaden the opening of the resist film downward along a thickness direction. Thus, the opening formed in the resist film has an upwardly decreasing width, or inwardly declined side walls.
As shown in FIG. 4B, a platinum thin film 2 is deposited on the whole surface. A platinum thin film 2 on the resist film 3 and a platinum thin film 2 on the insulating film 7 have stepwise cuts. The platinum thin film 2 on the insulating film 7 has an upwardly gradually thinned periphery. The resist film 3 is removed to lift off the unnecessary platinum thin film 2 on the resist film 3.
As shown in FIG. 4C, the platinum thin film pattern 2 is left.
With this second method, it is difficult to set the taper or slope degree of the side wall of the platinum thin film pattern with good controllability. Although the controllability can be improved by adopting a two-layer structure of the resist film 3, the number of processes is increased. If organic material such as resist is used, moisture compositions, unreacted compositions, dissolved compositions are produced from the resist film 3 when the platinum thin film 2 is deposited. These compositions adversely affect the quality of the platinum thin film. Since platinum has crystallinity, if impurities are mixed, the crystal growth becomes insufficient and the temperature characteristics are degraded. It is difficult to form the platinum thin film 2 having a good quality. If the resist film 3 is processed at a high temperature to remove these compositions beforehand, it becomes difficult to remove the resist film 3.